The present invention generally pertains to batteryless, portable frequency dividers such as are used as miniature signal-powered transponders in presence detection systems. Presence detection systems are useful for article surveillance and article-location determination. Batteryless, portable frequency dividers are described in U.S. Pat. No. 5,241,298 to Ming R. Lian and Fred W. Herman, U.S. Pat. No. 4,481,428 to Lincoln H. Chariot, Jr., U.S. Pat. No. 4,670,740 to Fred W. Herman and Lincoln H. Chariot, Jr. and U.S. Pat. No. 4,314,373 to Robert W. Sellers.
The frequency dividers described in U.S. Pat. Nos. 5,241,298; 4,481,428 and 4,314,373 each comprises a first parallel resonant circuit including an inductance and a capacitance that is resonant at a first frequency for receiving electromagnetic radiation at a first frequency and a second parallel resonant circuit including an inductance and a capacitance that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency.
In the frequency divider described in U.S. Pat. No. 5,241,298, the capacitance of one or both of the resonant circuits is a variable capacitance element in which the capacitance varies in accordance with the voltage across the variable capacitance element; and variation of the capacitance of the variable capacitance element in response to variations in energy in the first resonant circuit resulting from the first resonant circuit receiving electromagnetic radiation at the first frequency causes the second resonant circuit to transmit electromagnetic radiation at the second frequency The two resonant circuits are magnetically coupled to one another or electrically connected through an electrical coupling element, such as an additional coupling capacitor or a semiconductor element.
In the frequency divider described in U.S. Pat. No. 4,481,428 the two resonant circuits are electrically connected to one another by a semiconductor switching device that couples the first resonant circuit to the second resonant circuit to cause the second resonant circuit to transmit electromagnetic radiation at the second frequency in response to receipt of radiation at the first frequency. The resonant circuit inductances contain both in-phase and out-of-phase currents and the inductance cods are disposed perpendicular to each other so that the magnetic fields of the two coils are orthogonal in order to avoid cancellation of fields and a resulting decrease in efficiency.
In the frequency divider described in U.S. Pat. No. 4,314,373, the resonant circuits are coupled to one another through a variable capacitance element, such as a varactor diode, to cause the second resonant circuit to transmit electromagnetic radiation at the second frequency in response to receipt of electromagnetic radiation by the first resonant circuit at the first frequency.
The frequency divider described in U.S. Pat. No. 4,670,740 consists of a parallel resonant circuit including an inductance and variable capacitance device that is resonant at a second frequency that is one-half a first frequency to cause the circuit to transmit electromagnetic radiation at the second frequency in response to receipt of electromagnetic radiation at the first frequency.